1,2,4‐Oxadiazole‐Based Bent‐Core Liquid Crystals with Cybotactic Nematic Phases

Abstract

Several series of bent-core mesogens derived from 3,5-diphenyl-1,2,4-oxadiazole with or without lateral groups and with different length terminal chains at both ends, and polycatenar molecules with three to six alkoxy chains are synthesized and their mesomorphic behaviour is investigated by polarizing microscopy, differential scanning calorimetry, X-ray diffraction (XRD), dielectric, electro-optical and second-harmonic generation (SHG) experiments. Most compounds exhibit broad regions of skewed cybotactic nematic (NcybC ) and tilted smectic (SmC) phases with a strong tilt of the aromatic cores (up to 63°), but non-tilted SmA and NcybA phases are also observed for a compound that has only one terminal chain. The XRD patterns of the nematic phases of most of the compounds investigated indicate a 2D periodicity with short correlation length in the magnetically aligned samples. This is of importance for the general interpretation of the small-angle XRD splitting patterns typically observed for aligned samples of bent-core nematic phases. In most nematic phases one current peak is observed in the half period of an applied electric field, though no coherent signal is found in the SHG experiments. Based on additional electro-optical and dielectric results, the nematic phases are considered to be cybotactic nematic phases with local polar order, and show a dielectric reorientation of the polar domains. Only chiral nematic phases (NcybC *), but not blue phases, are obtained for compounds with one or two chiral (3S)-3,7-dimethyloctyloxy tail(s).

The effects of lateral halogen substituents on the low-temperature cybotactic nematic phase in oxadiazole based bent-core liquid crystals

“…[25] The cluster model has also been used to explain some other interesting features of BCM, such as the fact that chiral domains are observed when the sample is viewed by polarising microscopy (POM), [26][27][28][29][30][31][32] and ferroelectric switching behaviour in the nematic phase of 1,2,4-oxadiazolebased liquid crystals. [33][34][35][36][37] However, some differences of opinion regarding the precise cause of the observed behaviour in the nematic phase remain. [38][39][40][41] Clearly, there continues to be much to learn about the fundamentals of how the bent-core molecules are interacting to establish supramolecular structures in the nematic phase and this is being investigated using a variety of techniques.…”

Insights into Biaxial Ordering of Bent-Core Nematics: X-Ray Diffraction Evidence

“…the interaction with a field or a surface). Although this assumption was supported by several experimental findings [15,[21][22][23][24][26][27][28][29] and molecular simulations [15,30,31], there was in the past no direct experimental evidence of cluster biaxiality. In particular, XRD has long been considered unfit to probe biaxial order due to the inherently small molecular biaxiality of bent-core mesogens coupled with the typical broadness of X-ray scattering from nematics [2,4].…”

“…After a decade of experimental investigations, it is now widely recognized that the four-spot pattern is instead the signature of a cybotactic N phase, i.e., a N phase consisting of nano-sized molecular aggregates (the cybotactic clusters) characterized by tilted layered (i.e., smectic-C-like) positional order [10,[15][16][17][18][19][20][21]. In addition, cybotactic clusters are usually assumed to be inherently biaxial (and possibly polar [15,[22][23][24]), with cluster transverse axes randomly oriented with respect to neighboring clusters [25]. Hence, the cybotactic N phase is macroscopically uniaxial with a single common director, unless steps are taken to coherently align the biaxial clusters over large spatial scales with an external agent (e.g.…”

Flexoelectric polarization studies in bent-core nematic liquid crystals

“…We measure the sum of these coefficients for an ozadiazole systemss which do exhibit nematic phase over a wide range of temperatures. These are known to form clusters of various types and these systems have been extensively investigated over recent years using X-rays and N.M.R [26,27]. Figure 5 shows P f and a corresponding value of the pyroelectric coefficient γ. shows that these values range from 10 to 15 pC/m across the nematic region [27].…”